Move monzo_ynab into //tools

Optimizing is difficult: I like flat hierarchies because I don't like
directory-hopping, but I also would like a cleaner root for my mono-repo. Bombs
away!

Well it's that time again, folks: spring cleaning!

Here I am musing about a few things that bother me:
- Should I use kebab-case or snake_case?
- It feels ~confusing to have //tools and //utils. What a //projects? Isn't
  everything a project? *sigh*

tools/monzo_ynab/main.go

@@ -0,0 +1,43 @@
+// Exporting Monzo transactions to my YouNeedABudget.com (i.e. YNAB)
+// account. YNAB unfortunately doesn't currently offer an Monzo integration. As
+// a workaround and a practical excuse to learn Go, I decided to write one
+// myself.
+//
+// This job is going to run N times per 24 hours. Monzo offers webhooks for
+// reacting to certain types of events. I don't expect I'll need realtime data
+// for my YNAB integration. That may change, however, so it's worth noting.
+
+package main
+
+import (
+	"fmt"
+)
+
+var (
+	ynabAccountID = os.Getenv("ynab_account_id")
+)
+
+////////////////////////////////////////////////////////////////////////////////
+// Business Logic
+////////////////////////////////////////////////////////////////////////////////
+
+// Convert a Monzo transaction struct, `tx`, into a YNAB transaction struct.
+func toYnab(tx monzoSerde.Transaction) ynabSerde.Transaction {
+	return ynabSerde.Transaction{
+		Id: tx.Id,
+		Date: tx.Created,
+		Amount: tx.Amount,
+		Memo: tx.Notes,
+		AccountId: ynabAccountID,
+	}
+}
+
+func main() {
+	txs := monzo.TransactionsLast24Hours()
+	var ynabTxs []ynabSerde.Transaction{}
+	for tx := range txs {
+		append(ynabTxs, toYnab(tx))
+	}
+	ynab.PostTransactions(ynabTxs)
+	os.Exit(0)
+}